This invention relates to a controller for gas discharge lamps.
Gas discharge lamps such as fluorescent lamps, low pressure sodium or mercury vapour lamps, or high intensity discharge lamps (HID lamps) are normally operated with a so-called ballast device to control the power supply to the lamp. This is because such lamps have a negative impedance characteristic, that is, their impedances drop as the lamp current increases, necessitating control of the lamp current. In addition, such lamps normally require an ignition voltage which is considerably higher than the normal operating voltage. The standard practice has been to supply the gas discharge lamp with alternating current, and to use a series reactance to limit the current to the lamp. More recent developments include the use of electronic circuits to control the alternating current waveform supplied to the lamp. Circuits are known which control the duty cycle, amplitude or frequency of the waveform.
It is also possible to operate gas discharge lamps with direct current (DC). Early DC lamp controllers, such as those for carbon arc lamps, controlled the lamp current by means of a variable series resistance. The variable resistance was, in practice, required to dissipate approximately as much power as the lamp itself. Apart from this, DC operation caused unequal wear of the lamp electrodes, thus shortening lamp life or requiring special lamp design. Present gas discharge lamps are designed with electrodes of equal size, and would thus have a shortened life span if operated with conventional DC controllers. Despite the inefficiency of earlier DC controllers, DC operation of gas discharge lamps has particular advantages in certain applications. Where such lamps are used in motion picture filming, at sport stadiums or gymnasiums, or in factories, lamp flicker from 50 or 60 Hz mains supplies can cause stroboscopic effects which are undesirable and which may even be dangerous. In particular, the range of permissible motion picture filming speeds is severely curtailed. Flicker can also be avoided by high frequency AC operation, but the use of high frequencies can cause narrowing of the electron stream in the tube of the gas discharge lamp, causing it to move around and thus creating an irregular flicker. At certain operating frequencies, resonances may occur within the gas discharge lamp which cause a visible flicker or which may even cause the lamp to shatter.
German Offenlegungsschrift 28 19 003 describes a ballast circuit for fluorescent lamps in which an inverter supplies high frequency rectified AC to the lamp via a transformer circuit. A feedback circuit derives a DC feedback voltage which is used to modulate the waveform applied to the lamp, reducing the amplitude fluctuation in the pulsing DC supply to the inverter, and thus reducing the amplitude variation in the envelope of the high frequency lamp-driving waveform.
German Offenlegungsschrift 32 11 240 discloses a lamp controller for maintaining the light output of a lamp constant over its life span. An inverter powers the lamp via an impedance transforming network and a feedback signal maintains the lamp and the network in resonance. The circuit compensates for variations in the lamp impedance.
U.S. Pat. No. 4,277,728 discloses a circuit which supplies a gas discharge lamp with a sinusoidal waveform bby means of an inverter driving a resonant network which includes the lamp load.
According to U.S. Pat. No. 4,370,601, a gas discharge lamp is supplied with a relatively low voltage alternating current waveform, with a pulse generator being connected to the lamp circuit and supplying repeated ignition pulses to maintain the lamp in a conducting state.
The circuit described in European patent publication 0 127 101 shows an inverter-based controller, which supplies alternating current to a gas discharge lamp, the frequency of the current waveform varying according to the type of lamp used. A voltage sensor at the lamp determines the type of lamp in use and adjusts the operating frequency accordingly.
British patent specification GB 2,120,870 A describes a circuit in which an HF inverter feeds a gas discharge lamp via a resonant transformer circuit to provide a near sinusoidal waveform. The lamp light output is controlled by varying the duty cycle of the waveform.
U.K. patent application GB 2,124,045 A discloses a power amplifier for driving a gas discharge lamp, the amplifier being driven by an oscillator, and a current sensor and a gain control circuit maintaining the amplifier power output constant.